Interface synergism and engineering of Pd/Co@N-C for direct ethanol fuel cells

Direct ethanol fuel cells have been widely investigated as nontoxic and low-corrosive energy conversion devices with high energy and power densities. It is still challenging to develop high-activity and durable catalysts for a complete ethanol oxidation reaction on the anode and accelerated oxygen reduction reaction on the cathode. The materials’ physics and chemistry at the catalytic interface play a vital role in determining the overall performance of the catalysts. Herein, we propose a Pd/Co@N-C catalyst that can be used as a model system to study the synergism and engineering at the solid-solid interface. Particularly, the transformation of amorphous carbon to highly graphitic carbon promoted by cobalt nanoparticles helps achieve the spatial confinement effect, which prevents structural degradation of the catalysts. The strong catalyst-support and electronic effects at the interface between palladium and Co@N-C endow the electron-deficient state of palladium, which enhances the electron transfer and improved activity/durability. The Pd/Co@N-C delivers a maximum power density of 438 mW cm−2 in direct ethanol fuel cells and can be operated stably for more than 1000 hours. This work presents a strategy for the ingenious catalyst structural design that will promote the development of fuel cells and other sustainable energy-related technologies.

S-25 polarization curves of DEFC with an external reference Hg/HgO electrode. Pd/Co@N-C as both anode and cathode catalysts, the anode electrolyte is 1M KOH + 2 M EtOH with a flow rate of 5 mL min -1 , and the cathode was fed with oxygen with 200 mL min -1 without backpressure. (b) Faraday efficiency (FE) of EtOH to CO2 and acetate on Pd/C, Pd/N-C, Pd/Co@N-C, Pd+Co@N-C at 0.4 V vs. RHE. The error bars in (b) represent the s.d. of three independent tests, and the data are presented as mean values ±s.d. (c) The typical optical photograph of reacted electrolyte before and after adding excessive Ba(OH)2·8H2O to titrate the CO3 2for different catalysts after 3 hours i-t test at 0.4 V vs. RHE. After titration, a lot of white flocculent BaCO3 precipitations were obtained for Pd/Co@N-C sample, indicating that complete 12e pathway for EOR. While tiny turbid liquid was found for Pd/N-C and Pd+Co@N-C, indicating that a small amount of CO2 was generated on these two electrodes. In contrast, there is no change in the electrolytes with the Pd/C electrode, indicating that almost no CO2 was generated on Pd/C. (d) The 1 H-NMR results of different samples operated at 0.4 V vs. RHE for 3 hours. #1, #2, #3, and #4 in (c-d) represent Pd/C, Pd/N-C, Pd/Co@N-C, and Pd+Co@N-C catalysts, respectively. The peak at ~1.9 ppm is the characteristic peak of acetate (CH3COO -) due to the incomplete oxidation of EtOH. On Pd/C, Pd/N-C, and Pd+Co@N-C, this peak can be seen clearly, while no peaks at ~1.9 ppm can be found on Pd/Co@N-C indicating the complete EOR on Pd/Co@N-C.

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Supplementary Fig. 41. In-situ ATR-SEIRA spectra of Si prism in 0.1 M KOH with and without 1 M EtOH solutions. In 0.1 M KOH solution, no peak at ~1046 cm -1 can be found. In contrast, after 1 M EtOH was introduced, a sharp and strong peak at ~1046 cm -1 can be found clearly, which strongly confirmed that the 1046 cm -1 signal peak was come from EtOH, rather than from Si.

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Supplementary Fig. 42. In-situ ATR-SEIRA spectra measured at different potentials for Pd/Co@N-C in 0.1 M KOH + 1.0 M EtOH aqueous solution in the (a) window from 1400 to 2000 cm -1 , (b) CO2 spectral window from 2200 to 2500 cm -1 , and (c) CO spectral window from 1400 to 2000 cm -1 . The reference spectra in (a) (gray lines) were obtained at 0 V in 0.1 M KOH aqueous solution. The peak at 1430 cm -1 is attributed to the Au/Si background. To obtain the best spectroscopic results, the loading of 0.5 mgPd cm -2 was employed. The CO signal can be found at a much lower potential of 0.2 V vs RHE (Supplementary Fig. 42a) in the range of 1800-1828 cm -1 , and this characteristic CO peak was found at a wide potential of 0.2-0.9 V due to the continuous C-C bonds cleavage. The CO signal disappears above 0.9 V (Supplementary Figs. 42a, c) while the CO2 characteristic peak can be seen and became stronger as the potential increased from 1.0-1.4 V (Supplementary Fig. 42b), indicating that the C-C bond was easily cleaved at the low potential on Pd/Co@N-C and finally be oxidized to CO2 through a direct C1-12e pathway at a high potential. . After the potentiostatic i-t test for 12 h (0.5 V, 0.7 V, 0.9 V, and 1.1 V vs. RHE), 0.5 mL of electrolyte was used for the 13 C-NMR test immediately. The fresh electrolyte was also tested and used as a reference spectrum.

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Supplementary Fig. 44. In-situ ATR-SEIRA spectra measured at different potentials for Pt/C in 0.1 M KOH + 1.0 M EtOH aqueous solution. The reference spectra (black lines) were obtained at 0 V in 0.1 M KOH aqueous solution. The characteristic peak for the CO stretching vibration of C2H5OH at 1046 cm -1 was observed. The peak at 1417 cm -1 is attributed to the Au/Si background. The peak at 1650 cm -1 is attributed to the interfacial H2O. No CO characteristic peaks at ca. 1800-1828 cm -1 were found. As COads is the direct signal for the C-C bond cleavage during EOR complete oxidation, the results prove that the Pt/C samples show a powerless property for complete EOR, which results in an inferior activity than Pd/Co@N-C. were Pd/Co@N-C catalyst, which was strongly contacted with the membrane due to hot press during the preparation of MEA. (c) Specific area resistance (left axis) and specific conductivity (right axis) of AEM before and 1000 hours test. (d) CO stripping CV curves of Pd/Co@N-C after 1000 hours stability test. The CV curves were conducted in 0.1 M KOH at a scan rate of 20 mV s -1 . (e) ECSA (left axis) and CO oxidation peak potential (right axis) of Pd/Co@N-C before and after 1000 hours stability test, the ECSA was calculated from the charge integration of CO stripping that derived from (d) and Supplementary Fig. 24c. (f) CV curves for EOR before and after 1000 hours stability test in N2-saturated 1M KOH + 1M EtOH at a scan rate of 50 mV s -1 .

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The mass activity retention ratio of Pd/Co@N-C after 1000 hours of stability test was 89.1%, indicating the negligible effect of carbonate on the catalyst. The error bars in (c) and (e) represent the s.d. of three independent measurements, and the data were presented as mean values ±s.d.

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Supplementary Fig. 54. The movement of H2O and OHin the catalytic layer. (a) A piece of fresh carbon paper and (b) cathode catalytic layer (inset shows the corresponding pH) after 1000 hours stability test, a fully wetted cathode catalyst layer with a pH > 11 was found. (c) A schematic of DEFC that shows the movement of water (H2O) and hydroxide radical (OH -). The anode is fed with 1 M KOH + 2 M EtOH solution; thus, the water can move from the anode to the cathode through AEM (yellow arrows). The cathode was fed with 100% relative humidity of O2, which can ensure unrestrained movement of OHin the catalytic layer from cathode to anode (red arrows).

Supplementary Tables
Supplementary

Randles-Sevcik equation
The Randles-Sevcik equation is listed as follows: = (2.69 * 10 5 ) 3/2 1/2 1/2 (S14) where Ip is the peak current (A), n is the number of electrons participating in the reaction and here n = 1, A is the ECSA of the electrode (cm 2 ), D is the diffusion coefficient of K3[Fe(CN6)] (6.67*10 -6 cm -2 s -1 ), and C is the concentration of the molecule in the solution (mol L -1 ). Based on the above Randles-Sevcik equation, the ECSA (A, cm -2 ) is proportional to the value Ip/v 1/2 , which is Randles's slope shown in Supplementary Fig. 26e- (Supplementary Fig. 26k, right) for Pd/C, Pd/N-C, Pd/Co@N-C, Pd+Co@N-C, respectively. These values match well with the CO stripping results ( Supplementary Fig. 26k, left). The ECSA value calculated from the Randles-Sevcik equation (135.5 m 2 g -1 ) is a little higher than CO stripping method (117.9 m 2 g -1 ), thus the electronic effect indeed affects the CO adsorption on the catalyst surface and causes an underestimation of the ECSA of the as-prepared catalyst. While considering the same trend when calculating the ECSA with CO stripping and the Randles-Sevcik equation of all catalysts, it will not affect any conclusion made in this work.

Supplementary Note 3. Determination of potential ranges of EOR Faradic efficiency in three-electrode half cell
We monitored the anode potential during the fuel cell test using an external reference electrode (Int. J. Hydrogen Energy 2012Energy , 37, 2559Energy -2570J. Appl. Electrochem. 2013, 43, 1069-1078 to determine the range of the potential that should be applied in the three-electrode halfcell. As shown in Supplementary Fig. 34a too low potentials (<0.4 VRHE) give a very low current density. Thus, four potentials, that is 0.4, 0.5, 0.6, and 0.7 VRHE were selected to test the FE (Fig. 3f) for all four catalysts. The charge-toproduct balance from EtOH to carbonate ( Supplementary Fig. 34c) and acetate ( Supplementary   Fig. 34d) at 0.4 VRHE on the four electrodes has been verified and used as representative potential.
This result indicates that the OH-Pd(111) surface shows a higher EOR activity than Pd (111) surface via accelerating the rate-determining step of EOR, thus explaining the enhanced EOR activity of Pd/Co@N-C catalyst (Fig. 3).

Supplementary Note 5. The EOR activity of Au film and the ATR-SEIRAS of Au film
To prove that the Au layer has a negligible impact on Pd/Co@N-C when testing the ATR-SEIRAS, we do the following two experiments.
First, we tested the EOR performance of Au film. As can be seen in Supplementary Fig. 40a, the Au film electrode has EOR activity in an alkaline solution. However, the peak potential for EOR on Au film was located at ~1.2 VRHE. This potential is much higher than Pd/Co@N-C and other Pd-based reference catalysts (0.8-1.0 VRHE), and thus beyond the potential range that we studied. Besides, the EOR performance was compared in Supplementary Fig. 40b, the Au film electrode shows one order of magnitude lower current density (0.62 A mg -1 Au) than Pd/Co@N-C (7.06 A mg -1 Pd). Based on the much lower EOR activity and much higher oxidation potential of Au film than Pd/Co@N-C, it is believed that the Au film has negligible impact on the reported results.
Second, we further performed the ATR-SEIRAS test on the Au/Si prism for EOR in an alkaline solution. As can be seen from Supplementary Fig. 40c Based on the above, we believe that the peaks discussed are the information from Pd/Co@N-C catalyst, rather than from Au. The Au film/ disk electrode was widely used because it can provide a good signal-to-noise ratio (SNR), and thus the best spectroscopic result can be presented.
Supplementary Note 6. Confirmation of the C-C bond cleavage by isotopically labeled measurement.
The peak intensity at 171 ppm is always much higher than at 181 ppm from 0.5 V to 1.1 VRHE, indicating that the complete EOR is dominant on Pd/Co@N-C. The peak intensity reaches the maximum at 0.9 VRHE and decreases at 1.1 VRHE, which is consistent with the CV and i-t results.
Thus, the isotopically labeled measurement further confirmed that (i) 13 CO2 ( 13 CO3 2at 171 ppm) is the main EOR product; (ii) the peak intensity at 171 ppm is penitential-dependent, strongly demonstrates that the 13 CO3 2came from the complete oxidation of EtOH (1-13 C), rather than from contaminating of atmosphere or support corrosion.

Supplementary Note 7. Effect of carbonates produced from EOR on the membrane and catalyst
After 1000 hours of long-term stability test, the MEA was disassembled, and the reacted catalyst on the anode catalytic layer and anion-exchange membrane (AEM) were studied. The SEM images ( Supplementary Fig. 53a-b) show that the AEM pores were negligibly blocked after

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In addition, the specific area resistance and conductivity of the membrane just slightly increased and decreased respectively after 1000 hours test ( Supplementary Fig. 53c), further demonstrating that the CO3 2has an inappreciable effect on the membrane. It seems that most CO3 2was exist in the electrolyte, as we found that the pH of the initial electrolyte was 13.9, while it decreased to 13.1 after 1000 hours of stability test, which can be due to the consumption of OHthat reacted with the CO2. The decreased pH and the potential membrane blocking can be effectively prevented by replacing electrolytes periodically, increasing the flow rate of electrolytes, or replacing a new membrane, which has been confirmed by our recent work (Nature Energy, 2021, 6, 1144-1153. Besides, the effect of CO3 2on the catalyst was further evaluated. The CO stripping after 1000 hours of stability was tested to evaluate the ECSA and anti-CO poison ability of the reacted Pd/Co@N-C catalyst. the ECSA retention was 92.4% after 1000 hours test ( Supplementary Fig.   53d, left axis). While the CO oxidation peaks just have a positive of ca. 30 mV after 1000 hours stability (0.82 V) test compared with the fresh sample ( Supplementary Fig. 53d, right axis).
These results indicate that the carbonates (CO3 2-) may have a negative effect on reducing the active sites. However, due to the unique semi-embedded structure of Pd in the carbon layers, the catalytic activity for EOR ( Supplementary Fig. 53f) was well kept, even after 1000 hours of stability, the activity retentions for EOR are still 89.1% and matches well with the ECSA results.
In all, the carbonates (CO3 2-) that are produced from EOR just have negligible influence on the membrane and catalyst.
Supplementary Note 8. The movement of water (H2O) and hydroxide radical (OH -) in MEA and catalytic layer.
As can be seen from Supplementary Fig. 52, the thickness of the catalytic layer was about 74±2 μm. With such a thin layer, it has negligible mass and electron transfer resistance. Even though the KOH solution is not fed in the cathode, compared to the initial carbon paper ( Supplementary Fig. 54a), a fully wetted cathode catalyst layer (with a pH > 11, inset in S-57 Supplementary Fig. 54b) was found after 1000 hours long-term stability test, indicating that water can reach the entire cathode catalytic layer from the anode, as verified by the optical photograph shown in Supplementary Fig. 54b, this phenomenon matches well with recent review paper (Journal of Power Sources 2017, 341, 199-211) that water will transfer from anode to cathode ( Supplementary Fig. 54c). Besides, the AEM has been fully swelled with water before operation since it was stored in ultrapure water before use, and 100% relative humidity of O2 was fed to the cathode, which can ensure unrestrained movement of OHin the catalytic layer, and the fuel cells that using Nafion solution as binder can work smoothly.